NYS COMMON CORE MATHEMATICS CURRICULUM
Lesson 4
8•3
Lesson 4: Fundamental Theorem of Similarity (FTS)
Student Outcomes

Students experimentally verify the properties related to the fundamental theorem of similarity (FTS).
Lesson Notes
The goal of this activity is to show students the properties of the fundamental theorem of similarity (FTS) in terms of
dilation. FTS states that given a dilation from center 𝑂 and points 𝑃 and 𝑄 (points 𝑂, 𝑃, 𝑄 are not collinear), the
segments formed when 𝑃 is connected to 𝑄 and 𝑃′ is connected to 𝑄′ are parallel. More surprising is that
|𝑃′𝑄′| = 𝑟|𝑃𝑄|. That is, the segment 𝑃𝑄, even though it was not dilated as points 𝑃 and 𝑄 were, dilates to segment
𝑃′ 𝑄′ , and the length of segment 𝑃′𝑄′ is the length of segment 𝑃𝑄 multiplied by the scale factor. The following picture
refers to the activity suggested in the Classwork Discussion below. Also, consider showing the diagram (without the
lengths of segments), and ask students to make conjectures about the relationships between the lengths of segments
𝑃𝑄 and 𝑃′𝑄′.
Classwork
Discussion (30 minutes)
For this Discussion, students need a piece of lined paper, a centimeter ruler, a protractor, and a four-function (or
scientific) calculator.

The last few days, we have focused on dilation. We now want to use what we know about dilation to come to
some conclusions about the concept of similarity in general.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM

8•3
A regular piece of notebook paper can be a great tool for discussing similarity. What do you notice about the
lines on the notebook paper?

The lines on the notebook paper are parallel; that is, they never intersect.

Keep that information in mind as we proceed through this activity. On the first line of your paper, mark a point
𝑂. This is our center.

Mark the point 𝑃 a few lines down from the center 𝑂. From point 𝑂, draw a ray ⃗⃗⃗⃗⃗
𝑂𝑃 . Now, choose a 𝑃′ farther
down the ray, also on one of the lines of the notebook paper. For example, you may have placed point 𝑃 three
lines down from the center and point 𝑃′ five lines down from the center.

Use the definition of dilation to describe the lengths along this ray.


By the definition of dilation, |𝑂𝑃′| = 𝑟|𝑂𝑃|.
Recall that we can calculate the scale factor using the following computation:
using the lines on the paper as our unit, the scale factor is 𝑟 =
|𝑂𝑃′ |
|𝑂𝑃|
= 𝑟. In my example,
5
because point 𝑃′ is five lines down from the
3
center, and point 𝑃 is three lines down from the center. On the top of your paper, write the scale factor that
you have obtained.

Now draw another ray, ⃗⃗⃗⃗⃗⃗
𝑂𝑄 . Use the same scale factor to mark points 𝑄 and 𝑄′. In my example, I would place
𝑄 three lines down and 𝑄′ five lines down from the center.

Now connect point 𝑃 to point 𝑄 and point 𝑃′ to point 𝑄′. What do you notice about lines 𝑃𝑄 and 𝑃′𝑄′?


Use your protractor to measure ∠𝑂𝑃𝑄 and ∠𝑂𝑃′𝑄′. What do you notice, and why is it so?


∠𝑂𝑃𝑄 and ∠𝑂𝑃′𝑄′ are equal in measure. They must be equal in measure because they are
corresponding angles of parallel lines (lines 𝑃𝑄 and 𝑃′𝑄′) cut by a transversal (ray ⃗⃗⃗⃗⃗
𝑂𝑃 ).
(Consider asking students to write their answers to the following question in their notebooks and to justify
their answers.) Now, without using your protractor, what can you say about ∠𝑂𝑄𝑃 and ∠𝑂𝑄′𝑃′?


The lines 𝑃𝑄 and 𝑃′𝑄′ fall on the notebook lines, which means that lines 𝑃𝑄 and 𝑃′𝑄′ are parallel lines.
These angles are also equal for the same reason; they are corresponding angles of parallel lines (lines
⃗⃗⃗⃗⃗⃗ ).
𝑃𝑄 and 𝑃′𝑄′) cut by a transversal (ray 𝑂𝑄
Use your centimeter ruler to measure the lengths of segments 𝑂𝑃 and 𝑂𝑃′. By
the definition of dilation, we expect |𝑂𝑃′| = 𝑟|𝑂𝑃| (i.e., we expect the length of
segment 𝑂𝑃′ to be equal to the scale factor times the length of segment 𝑂𝑃).
Verify that this is true. Do the same for lengths of segments 𝑂𝑄 and 𝑂𝑄′.

Sample of what student work may look like:
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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This file derived from G8-M3-TE-1.3.0-08.2015
Note to Teacher:
Using a centimeter ruler makes
it easier for students to come
up with a precise
measurement. Also, let
students know that it is okay if
their measurements are off by
a tenth of a centimeter,
because that difference can be
attributed to human error.
45
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NYS COMMON CORE MATHEMATICS CURRICULUM

Lesson 4
8•3
Bear in mind that we have dilated points 𝑃 and 𝑄 from center 𝑂 along their respective rays. Do you expect the
segments 𝑃𝑄 and 𝑃′𝑄′ to have the relationship |𝑃′𝑄′| = 𝑟|𝑃𝑄|?

(Some students may say yes. If they do, ask for a convincing argument. At this point, they have
knowledge of dilating segments, but that is not what we have done here. We have dilated points and
then connected them to draw the segments.)

Measure the segments 𝑃𝑄 and 𝑃′𝑄′ to see if they have the relationship |𝑃′𝑄′| = 𝑟|𝑃𝑄|.

It should be somewhat surprising that, in fact, segments 𝑃𝑄 and 𝑃′𝑄′ enjoy the same properties as the
segments that we actually dilated.

Now mark a point 𝐴 on line 𝑃𝑄 between points 𝑃 and 𝑄. Draw a ray from center 𝑂 through point 𝐴, and then
mark point 𝐴′ on the line 𝑃′𝑄′. Do you think |𝑃′𝐴′| = 𝑟|𝑃𝐴|? Measure the segments, and use your calculator
to check.


Now mark a point 𝐵 on the line 𝑃𝑄 but this time not on the segment 𝑃𝑄 (i.e., not between points 𝑃 and 𝑄).
Again, draw the ray from center 𝑂 through point 𝐵, and mark the point 𝐵′ on the line 𝑃′𝑄′. Select any of the
̅̅̅̅, and verify that it has the same property as the others.
̅̅̅̅ , or 𝑄𝐵
segments, ̅̅̅̅
𝐴𝐵 , 𝑃𝐵


Sample of what student work may look like:
Does this always happen, no matter the scale factor or placement of points 𝑃, 𝑄, 𝐴, and 𝐵?


Students should notice that these new segments also have the same properties as the dilated segments.
Yes, I believe this is true. One main reason is that everyone in class probably picked different points,
and I’m sure many of us used different scale factors.
In your own words, describe the rule or pattern that we have discovered.
MP.8 Encourage students to write and collaborate with a partner to answer this question. Once students have finished their
work, lead a discussion that crystallizes the information in the theorem that follows.

We have just experimentally verified the properties of the fundamental theorem of similarity (FTS) in terms of
dilation, namely, that the parallel line segments connecting dilated points are related by the same scale factor
as the segments that are dilated.
THEOREM: Given a dilation with center 𝑂 and scale factor 𝑟, then for any two points 𝑃 and 𝑄 in the plane so
that 𝑂, 𝑃, and 𝑄 are not collinear, the lines 𝑃𝑄 and 𝑃′𝑄′ are parallel, where 𝑃′ = 𝐷𝑖𝑙𝑎𝑡𝑖𝑜𝑛(𝑃) and
𝑄′ = 𝐷𝑖𝑙𝑎𝑡𝑖𝑜𝑛(𝑄), and furthermore, |𝑃′𝑄′| = 𝑟|𝑃𝑄|.
Ask students to paraphrase the theorem in their own words, or offer them the following version of the theorem: FTS
states that given a dilation from center 𝑂 and points 𝑃 and 𝑄 (points 𝑂, 𝑃, and 𝑄 are not on the same line), the
segments formed when you connect 𝑃 to 𝑄 and 𝑃′ to 𝑄′ are parallel. More surprising is the fact that the segment 𝑃𝑄,
even though it was not dilated as points 𝑃 and 𝑄 were, dilates to segment 𝑃′ 𝑄′ , and the length of segment 𝑃′𝑄′ is the
length of segment 𝑃𝑄 multiplied by the scale factor.

Now that we are more familiar with properties of dilations and FTS, we begin using these properties in the next
few lessons to do things like verify similarity of figures.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
8•3
Exercise (5 minutes)
Exercise
In the diagram below, points 𝑹 and 𝑺 have been dilated from center 𝑶 by a scale factor of 𝒓 = 𝟑.
a.
If |𝑶𝑹| = 𝟐. 𝟑 𝐜𝐦, what is |𝑶𝑹′ |?
|𝑶𝑹′ | = 𝟑(𝟐. 𝟑 𝐜𝐦) = 𝟔. 𝟗 𝐜𝐦
b.
If |𝑶𝑺| = 𝟑. 𝟓 𝐜𝐦, what is |𝑶𝑺′ |?
|𝑶𝑺′ | = 𝟑(𝟑. 𝟓 𝐜𝐦) = 𝟏𝟎. 𝟓 𝐜𝐦
c.
Connect the point 𝑹 to the point 𝑺 and the point 𝑹′ to the point 𝑺′. What do you know about the lines that
contain segments 𝑹𝑺 and 𝑹′ 𝑺′?
The lines containing the segments 𝑹𝑺 and 𝑹′𝑺′ are parallel.
d.
What is the relationship between the length of segment 𝑹𝑺 and the length of segment 𝑹′ 𝑺′?
The length of segment 𝑹′ 𝑺′ is equal to the length of segment 𝑹𝑺, times the scale factor of 𝟑 (i.e.,
|𝑹′ 𝑺′ | = 𝟑|𝑹𝑺|).
e.
Identify pairs of angles that are equal in measure. How do you know they are equal?
|∠𝑶𝑹𝑺| = |∠𝑶𝑹′ 𝑺′ | and |∠𝑶𝑺𝑹| = |∠𝑶𝑺′ 𝑹′ | They are equal because they are corresponding angles of
parallel lines cut by a transversal.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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NYS COMMON CORE MATHEMATICS CURRICULUM
Lesson 4
8•3
Closing (5 minutes)
Summarize, or ask students to summarize, the main points from the lesson.

We know that the following is true: If |𝑂𝑃′| = 𝑟|𝑂𝑃| and |𝑂𝑄′| = 𝑟|𝑂𝑄|, then |𝑃′𝑄′| = 𝑟|𝑃𝑄|. In other
words, under a dilation from a center with scale factor 𝑟, a segment multiplied by the scale factor results in the
length of the dilated segment.

We also know that the lines 𝑃𝑄 and 𝑃′𝑄′ are parallel.

We verified the fundamental theorem of similarity in terms of dilation using an experiment with notebook
paper.
Lesson Summary
THEOREM: Given a dilation with center 𝑶 and scale factor 𝒓, then for any two points 𝑷 and 𝑸 in the plane so that 𝑶,
𝑷, and 𝑸 are not collinear, the lines 𝑷𝑸 and 𝑷′𝑸′ are parallel, where 𝑷′ = 𝑫𝒊𝒍𝒂𝒕𝒊𝒐𝒏(𝑷) and 𝑸′ = 𝑫𝒊𝒍𝒂𝒕𝒊𝒐𝒏(𝑸),
and furthermore, |𝑷′𝑸′| = 𝒓|𝑷𝑸|.
Exit Ticket (5 minutes)
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
Name ___________________________________________________
8•3
Date____________________
Lesson 4: Fundamental Theorem of Similarity (FTS)
Exit Ticket
Steven sketched the following diagram on graph paper. He dilated points 𝐵 and 𝐶 from point 𝑂. Answer the following
questions based on his drawing.
1.
What is the scale factor 𝑟? Show your work.
2.
Verify the scale factor with a different set of segments.
3.
Which segments are parallel? How do you know?
4.
Are ∠𝑂𝐵𝐶 and ∠𝑂𝐵′𝐶′ right angles? How do you know?
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
8•3
Exit Ticket Sample Solutions
Steven sketched the following diagram on graph paper. He dilated points 𝑩 and 𝑪 from point 𝑶. Answer the following
questions based on his drawing.
1.
What is the scale factor 𝒓? Show your work.
|𝑶𝑩′ | = 𝒓|𝑶𝑩|
𝟕 =𝒓⋅𝟑
𝟕
=𝒓
𝟑
2.
Verify the scale factor with a different set of segments.
|𝑩′ 𝑪′ | = 𝒓|𝑩𝑪|
𝟕=𝒓⋅𝟑
𝟕
=𝒓
𝟑
3.
Which segments are parallel? How do you know?
Segments 𝑩𝑪 and 𝑩′𝑪′ are parallel since they lie on the grid lines of the paper, which are parallel.
4.
Are ∠𝑶𝑩𝑪 and ∠𝑶𝑩′𝑪′ right angles? How do you know?
The grid lines on graph paper are perpendicular, and since perpendicular lines form right angles, ∠𝑶𝑩𝑪 and ∠𝑶𝑩′𝑪′
are right angles.
Problem Set Sample Solutions
Students verify that the fundamental theorem of similarity holds true when the scale factor 𝑟 is 0 < 𝑟 < 1.
1.
Use a piece of notebook paper to verify the fundamental theorem of similarity for a scale factor 𝒓 that is
𝟎 < 𝒓 < 𝟏.

Mark a point 𝑶 on the first line of notebook paper.

⃗⃗⃗⃗⃗⃗ . Mark the point 𝑷′ on the
Mark the point 𝑷 on a line several lines down from the center 𝑶. Draw a ray, 𝑶𝑷
ray and on a line of the notebook paper closer to 𝑶 than you placed point 𝑷. This ensures that you have a
scale factor that is 𝟎 < 𝒓 < 𝟏. Write your scale factor at the top of the notebook paper.

⃗⃗⃗⃗⃗⃗ , and mark the points 𝑸 and 𝑸′ according to your scale factor.
Draw another ray, 𝑶𝑸

Connect points 𝑷 and 𝑸. Then, connect points 𝑷′ and 𝑸′.

⃗⃗⃗⃗⃗⃗ . Mark point 𝑨′ at
Place a point, 𝑨, on the line containing segment 𝑷𝑸 between points 𝑷 and 𝑸. Draw ray 𝑶𝑨
the intersection of the line containing segment 𝑷′𝑸′ and ray ⃗⃗⃗⃗⃗⃗
𝑶𝑨.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
8•3
Sample student work is shown in the picture below:
a.
Are the lines containing segments 𝑷𝑸 and 𝑷′𝑸′ parallel lines? How do you know?
Yes, the lines containing segments 𝑷𝑸 and 𝑷′𝑸′ are parallel. The notebook lines are parallel, and these lines
fall on the notebook lines.
b.
Which, if any, of the following pairs of angles are equal in measure? Explain.
i.
∠𝑶𝑷𝑸 and ∠𝑶𝑷′𝑸′
ii.
∠𝑶𝑨𝑸 and ∠𝑶𝑨′𝑸′
iii.
∠𝑶𝑨𝑷 and ∠𝑶𝑨′𝑷′
iv.
∠𝑶𝑸𝑷 and ∠𝑶𝑸′𝑷′
All four pairs of angles are equal in measure because each pair of angles are corresponding angles of parallel
lines cut by a transversal. In each case, the parallel lines are line 𝑷𝑸 and line 𝑷′ 𝑸′ , and the transversal is the
respective ray.
c.
Which, if any, of the following statements are true? Show your work to verify or dispute each statement.
i.
|𝑶𝑷′| = 𝒓|𝑶𝑷|
ii.
|𝑶𝑸′| = 𝒓|𝑶𝑸|
iii.
|𝑷′𝑨′| = 𝒓|𝑷𝑨|
iv.
|𝑨′𝑸′| = 𝒓|𝑨𝑸|
All four of the statements are true. Verify that students have shown that the length of the dilated segment
was equal to the scale factor multiplied by the original segment length.
d.
Do you believe that the fundamental theorem of similarity (FTS) is true even when the scale factor is 𝟎 < 𝒓 <
𝟏? Explain.
Yes, because I just experimentally verified the properties of FTS for when the scale factor is 𝟎 < 𝒓 < 𝟏.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
2.
8•3
Caleb sketched the following diagram on graph paper. He dilated points 𝑩 and 𝑪 from center 𝑶.
a.
What is the scale factor 𝒓? Show your work.
|𝑶𝑩′ | = 𝒓|𝑶𝑩|
𝟐 =𝒓⋅𝟔
𝟐
=𝒓
𝟔
𝟏
=𝒓
𝟑
b.
Verify the scale factor with a different set of segments.
|𝑩′ 𝑪′ | = 𝒓|𝑩𝑪|
𝟑 =𝒓⋅𝟗
𝟑
=𝒓
𝟗
𝟏
=𝒓
𝟑
c.
Which segments are parallel? How do you know?
Segment 𝑩𝑪 and 𝑩′𝑪′ are parallel. They lie on the lines of the graph paper, which are parallel.
d.
Which angles are equal in measure? How do you know?
|∠𝑶𝑩′ 𝑪′ | = |∠𝑶𝑩𝑪|, and |∠𝑶𝑪′ 𝑩′ | = |∠𝑶𝑪𝑩| because they are corresponding angles of parallel lines cut by
a transversal.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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Lesson 4
NYS COMMON CORE MATHEMATICS CURRICULUM
3.
8•3
Points 𝑩 and 𝑪 were dilated from center 𝑶.
a.
What is the scale factor 𝒓? Show your work.
|𝑶𝑪′ | = 𝒓|𝑶𝑪|
𝟔= 𝒓⋅𝟑
𝟔
=𝒓
𝟑
𝟐=𝒓
b.
If |𝑶𝑩| = 𝟓, what is |𝑶𝑩′ |?
|𝑶𝑩′ | = 𝒓|𝑶𝑩|
|𝑶𝑩′ | = 𝟐 ⋅ 𝟓
|𝑶𝑩′ | = 𝟏𝟎
c.
How does the perimeter of triangle 𝑶𝑩𝑪 compare to the perimeter of triangle 𝑶𝑩′𝑪′?
The perimeter of triangle 𝑶𝑩𝑪 is 𝟏𝟐 units, and the perimeter of triangle 𝑶𝑩′𝑪′ is 𝟐𝟒 units.
d.
Did the perimeter of triangle 𝑶𝑩′𝑪′ = 𝒓 × (perimeter of triangle 𝑶𝑩𝑪)? Explain.
Yes, the perimeter of triangle 𝑶𝑩′𝑪′ was twice the perimeter of triangle 𝑶𝑩𝑪, which makes sense because
the dilation increased the length of each segment by a scale factor of 𝟐. That means that each side of triangle
𝑶𝑩′𝑪′ was twice as long as each side of triangle 𝑶𝑩𝑪.
Lesson 4:
Fundamental Theorem of Similarity (FTS)
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